Plant fibers

Fibers from plant materials can be used for sustainable horticulture. Examples are wood chips, wood shavings, woody fractions of peat bogs, woody fraction of compost, miscanthus, bark, flax shives, reed, etcetera. If you are interested in a specific plant material, click on this box to see the specific opportunities.

Plant fibers can be treated by defiberization and by gasification. Click on the specific boxes for more information on techniques, necessary conditions, tests, etcetera.

Woody fraction of green waste is treated as a separate stream and has also several treatment options for sustainable horticulture (see box below). 

Woody fraction of green waste

Woody fraction of green waste can be treated for sustainable horticulture, by defiberation and gasification. Click on the specific boxes for more information on techniques, necessary conditions, tests, etcetera.

Spent growing media

Spent growing media can be treated for sustainable horticulture by gasification, composting and steam treatment. Click on the specific boxes for more information on techniques, necessary conditions, tests, etcetera.

 

Shellfish waste

Seafood waste material can be treated for sustainable horticulture. Examples are shrimp shells, crab shells and Chines mitten crab. If you are interested in a specific shellfish waste, click on this box to see the specific opportunities.

Shellfish waste can be treated by torrefication and chitin extraction. Click on the specific boxes for more information on techniques, necessary conditions, tests, etcetera.

 

Gases

Gases containing CO2 can be purified for use in sustainable horticulture. All type of flue gases with sufficient amount of CO2 are in general suitable for the technology. Test and full scale industrial units are operational using flue gases from wood fired boilers.

Wood chips

Wood chips, e.g. from the wood industry. Wood chips were tested for defiberization by hammer mill and wood chipper and for gasification (click on the boxes of these techniques for more information).

Wood shavings

Wood shavings, e.g. from the wood industry. Wood shavings were tested for defiberization by hammer mill (click on the boxes of these techniques for more information).

Woody fraction peat bogs

Woody fraction sieved from peat bogs were tested for defiberization by hammer mill and extruder (click on the boxes of these techniques for more information).

Woody fraction compost

Woody fraction of green waste compost was tested for defiberization by hammer mill and extruder (click on the boxes of these techniques for more information).

Woody fraction of green waste

(tekst nog aan te leveren)

Miscanthus

Miscanthus shives were tested for defiberization by hammer mill and extruder, and for gasification (click on the boxes of these techniques for more information).

Bark

Bark from Pinus sylvestris and Pinus maritima was tested for defiberization by hammer mill (click on the boxes of these techniques for more information).

Flax shives

Flax shives were tested for defiberization by hammer mill and for gasification (click on the boxes of these techniques for more information).

Reed

Reed was not specifically tested in the Horti-BlueC project, but it was tested for defiberization and other treatments in the I-Love-T project (Vandecasteele et al., 2018). A special attention point is the moist content that should not be too high.

Vandecasteele, B., Muylle, H., De Windt, I., Van Acker, J., Ameloot, N., Moreaux, K., Coucke, P. and Debode, J. (2018) Plant fibers for renewable growing media: Potential of defibration, acidification or inoculation with biocontrol fungi to reduce the N drawdown and plant pathogens. J. Clean Prod. 203, 1143-1154

Organic growing media

Only spent organic growing media (based on peat, coir, plant fibers, compost, etcetera) can be treated by gasification, composting and steam treatment (click on the boxes of these techniques for more information).

Mineral growing media

Spent mineral growing media (e.g. based on stone wool) can not be treated by gasification, composting and steam treatment.

Shrimp shells

Shrimp shells were tested for chitin extraction and torrefication (click on the boxes of these techniques for more information).

Crab shells

Crab shells were not specifically tested within the Horti-BlueC project, but could similar to the shrimp shells by treated by chitin extraction and torrefication (click on the boxes of these techniques for more information).

Chinese mitten crab

The invasive species Chinese mitten crab was tested for chitin extraction and could potentially also be used for torrefication (click on the boxes of these techniques for more information).

Defiberization

During this technique plant fibers are subjected to high pressure and high temperature treatments, rendering a light and fluffy material, with a high internal surface, ideal for the use in growing media. In addition the defiberization technique renders the plant fibers less susceptible to N immobilization when used in the growing media.

Necessary conditions of the material:

  • To run a successful test, a minimum of 5 m³ material is needed.
  • No rocks may be present in the materials.
  • Storage: The treated materials can be stored up to a year when stored dry, and when the temperatures are checked weekly to avoid composting inside the material. Especially materials with small fraction size (0-10 mm) can start a composting process when stored in heaps. The temperature must be checked weekly to see whether the heap must be opened and cooled to prevent composting.

Gasification

Pyrolysis is the general term that refers to the process of heating biomass to temperatures typically between 300°C and 700°C under oxygen-deprived conditions, leading to biochar as the solid co-product together with combustible gas. The addition of sub stoichiometric oxygen changes the process to gasification, that leads to less biochar and more combustible gas yield, and more important, leads to biochars with higher porosity and very low tar content.

The waste streams need to be placed in an oven for drying prior to their storage, to avoid decay yeast formation before their thermal treatment. There is no maximum period of storage, but storage must be cool and dry.

During the pyrolysis, combustible gas is formed which is intended for heat production in gas boilers or combustors. Special care must be taken to avoid leaking of gas during the process. The standard safety equipment (gas monitoring, gas pipelines, fire security) with which the lab is equipped is sufficient to conform to safety rules.

Concerning the waste stream processing, the standard personal safety equipment (gloves, glasses) is sufficient, as the streams are not dangerous or toxic.

There are no legal aspects that may create obstacles as to the operation of the waste handling and the use of products.

Biochar quantities of approximately 3 kg have been produced for each processing condition and feedstock, and samples have been provided to ILVO for initial analysis of their agronomical and chemical analysis.

Upscale of the process has not been possible at the moment. At industrial scale probably more than one reactor has to operate in parallel to achieve the necessary quantity.

Pyrolysis experiments have been performed by ECN > TNO using the Pyromaat reactor under controlled conditions. The Pyromaat reactor is an indirectly heated screw feeding reactor. It is able to convert typically 3 - 5 kg/h of feestock in an O2-free atmosphere at temperatures up to 800°C. Solid fuel residence times are 15 – 30 and 60 minutes, steam and CO2 was used for activation. The heat needed in the rotary kiln reactor is supplied indirectly over the reactor wall. The pyrolysis gas temperature at the outlet of the rotary kiln can be set between 350° C and 700° C. The reactor is a cylindrical horizontal vessel, which rotates slowly around its axis. As the reactor rotates, material gradually moves down towards the lower end, and may undergo a certain amount of stirring and mixing. This reactor is typical used for low temperature pyrolysis because of the longer residence times and an unproblematic continuous feeding and discharge of the solid components. The reactor allows for an inert gas flow in countercurrent mode to prevent tar condensation.

The pyrolysis experiments include the named waste streams at a fixed furnace temperatures (650 °C and 400oC as measured at the outer wall of the reactor). There are numerous temperature measurement points on the outside wall (T1-10) but not on the inside wall of the reactor tube (shown in Fig. 3). Each pyrolysis run lasted at least three hour to ensure sucient biochar product for characterisation and incubation experiments. This first test series focused on the complete energy and mass balances of the biochar production process in order to identify bottlenecks and thus optimise the process. Rather than delivering separate factsheets per feedstock we chose to deliver one factsheet for this first and complete test round, while fact sheet 2 focuses on the actual production of quantities per feedstock without further focus on the full mass and energy balance.

In a second set of experiments, the pyrolysis conditions remained at 400°C in order to focus on the tradeoff between biochar yield and quality. The pyrolysis experiments include the named waste streams at a fixed furnace temperatures (650 °C as measured at the outer wall of the reactor). There are numerous temperature measurement points on the outside wall (T1-10) but not on the inside wall of the reactor tube (shown in Fig. 2). Each pyrolysis run lasted at least three hour to ensure sucient biochar product for characterisation and incubation experiments.



(a)

(b)

Figure 3.  (a) Full view of the Pyromaat and (b)  schema of the counter current flow

 

Results for the first experiments:

-          Biochar and combustible gas: Mass yield shown in Figure 1

-          Energy and Mass balance: Results in Figure 2

 

Figure 1. Biochar and co- product gas yield based on feed input Woody fraction of green waste = Park residues / Plant fibers = Miscanthus, Flax shives / Spent growing media = coco coir, spent peat

 

From Figure 1 one can observe that at 400oC the biochar yield is considerably higher than at 650oC. This has an impact on the properties of the biochars at those temperatures, with the lower T biochars being less stable than biochars at higher temperatures. However for the purpose of using them as peat substrates this difference may not be a decisive factor. The biochar properties are shown in Table 2 in the Annex.

Figure 2 focuses on a detailed energy balance, where some interesting observations are made. The difference between the red and blue line gives the heat in the produced gas which is available for further use, e.g. greenhouse heating. This availability fluctuates with the feedstock, the high ash fuels require a lot of heat that dissipates in the ash (inert) while high quality woody biomass requires heat for the pyrolysis but releases combustible gases that are valorised further downstream.

 

 Figure 2. Required vs. available energy balance

 The higher processing temperatures promote secondary reactions involving strong thermal cracking and reforming, resulting in an increase in permanent gas yield

The high N2 content in the product gas in some of the tests may pose a challenge for its potential use in a gas combustion system, as significant volumes of gas are needed to replace e.g. natural gas firing. Furthermore, because these gases are usually available at only a very low pump pressure, lean gases often cannot be utilized thermally. Instead they are flared off. Special burners exist though, that feature a low pressure requirement of only ~10 mbar. The energy balance shown in Figure 2, indicates that the energy content of the by-produced gas from the pyrolysis process is sufficient to cover the energy needs for the whole pyrolysis reactions, including the warming up and steam production, as well as the endothermic reactions that take place. Therefore we conclude that the energy valorization of the product gas is feasible and cover the thermal needs of the installation.

 Results for the second set of experiments:

-          Biochar and combustible gas: Mass yield shown in Figure 1

 

Figure 1. Biochar yield (mass %) for the second series of biochar production

 

Woody fraction of green waste = Park residues / Plant fibers = Miscanthus, Flax shives / Spent growing media = coco coir, spent peat

 The high N2 content in the product gas in some of the tests may pose a challenge for its potential use in a gas combustion system, as significant volumes of gas are needed to replace e.g. natural gas firing. Furthermore, because these gases are usually available at only a very low pump pressure, lean gases often cannot be utilized thermally. Instead they are flared off. Special burners exist though, that feature a low pressure requirement of only ~10 mbar. The energy balance shown in Figure 2, indicates that the energy content of the by-produced gas from the pyrolysis process is sufficient to cover the energy needs for the whole pyrolysis reactions, including the warming up and steam production, as well as the endothermic reactions that take place. Therefore we conclude that the energy valorization of the product gas is feasible and cover the thermal needs of the installation.

 Conclusions:

The high ash content feedstocks may be challenging in running a pyrolysis process self sustaining energetically, as the released combustible gas may be marginally sufficient for running the endothermal pyrolysis process and at the same time deliver surplus energy for heating.

The lower temperature pyrolysis leads to higher yields, as we can conclude by comparing with Figure 1.

 Figure 2 summarizes the biochar yield per feedstock for the two temperatures, 400 and 650oC

Figure 2. Biochar yield (%) in all the tests

 

Composting

Spent growing media can be composted by the grower, or at a certified composting facility. It is better to avoid too wet storage conditions.

Spent growing media used in strawberry and tomato cultivation were used as bulking agent (BA) for processing leek residues in windrow composting. The compost trial was executed in open air on a concrete pad at the composting facility (compost pilot) of the experimental farm of the Institute for Agricultural and Fisheries Research (ILVO) in Merelbeke, Belgium. The compost piles were covered with a geotextile (TopTex® TenCate) to protect them from precipitation. During the composting process, temperature and CO2 levels were monitored at least twice per week. When the critical temperature and/or CO2 value (65°C and/or 16 vol% CO2) were reached, the piles were aerated by turning with the TG 301 Gujer Innotec compost turner and water was added when the mixture became too dry  to avoid suboptimal process conditions.

The strawberry substrate consisted of 50% peat, 35% coconut fiber and 15% perlite, the tomato substrate consisted of 40% peat, 20% coconut fiber and 40% peat moss. Two windrows were set up; in the beginning of the process, each windrow was 12 m long, 3 m wide and 1.5 m high. Each windrow consisted of 200 kg of fresh straw and 4225 kg fresh weight (FW) of leek crop residues. Each windrow contained an equal volume (20.8 m³) of the BA, i.e., spent strawberry or tomato substrate. The strawberry substrate consisted of 50% peat, 35% coconut fiber and 15% perlite, the tomato substrate consisted of 40% peat, 20% coconut fiber and 40% peat moss.


Compared to wood chips, spent strawberry and tomato substrates led to lower pile temperatures (max. 42°C) during the first nine days of composting. It is supposed that this was a pure physical effect due to the pores being blocked by the dense structure of the substrates. Aeration was lower, leading to anaerobic conditions after intense microbial activity, resulting in high CO2 concentrations, that subsequently hindered the further growth of aerobic microorganisms and thus the composting process. Turning the pile and adding a coarser BA (wood bark) reduced the density of the mixture, resulting in lower CO2 concentrations and thus increased oxygen availability. Additionally, the initial substrates and mixtures at the start were less biodegradable (lower OUR and biodegradation potential) compared to the wood chips mixtures (reference treatment). This is related to the recalcitrant C in the peat (the major component of the substrates). Only a small decrease in OM content and C/N ratio during the process were measured for the spent strawberry and tomato substrates used as BA. 

When using spent growing substrates, the compost requires more frequent turning in comparison with wood chips due to oxygen shortage in these dense mixtures. To prevent oxygen shortage when applying these alternative BA, a coarser BA (i.e., larger particle size) with more structure could be added. Due to the suboptimal feedstock mixtures, pile temperatures did not surpass the required temperatures to ensure compost sanitation. Use of spent growing media resulted in lower OM degradation, because the substrates had a lower biodegradation potential related to the high content of recalcitrant C in the peat. Spent growing media in composting result in high CO2 concentrations. To be tested in the future together with other bulking agents.



 



Steam treatment

Steam treatment was not specifically tested in the Horti-BlueC project, but this treatment was successfully tested in the ReGrow4C project 

Chitin extraction

Chitin is a biopolymer that can be extracted from shellfish waste. The extraction can be chemically and enzymatically be performed (click on the boxes of these techniques for more information).

Torrefaction

Dry torrefaction, a mild temperature treatment at 250 -320oC in the absence of oxygen, is an upgrading technology that increases the biomass energy density and grindability and thus enhances fuel quality and biologic stability while reduces the logistics costs related to transport and storage per energy unit.

An alternative pretreatment (next to chitin extraction) of the shrimp shells includes a dry roasting process. It is a relatively cheap alternative to the chemical chitin extraction process. The main aim is to preserve the chitin but to sanitize the material and make it more easy to store for longer periods of time.

The batch size fixed bed torrefaction reactor is shown in Picture 1. It is directly heated and typically handles 3 - 5 kg per feedstock/batch. The reactor consists of a vertical cylinder with and internal diameter (i.d.) of 16 cm and effective length of 1 m. The bed is divided in three sections to distinguish possible differences in torrefaction behaviour over the bed height. A gas distribution plate supports each section. The reactor is directly heated by supplying preheated nitrogen through a distributor plate at the bottom. In addition, trace heating is applied to compensate for the relatively large heat losses via the reactor vessel for this size of equipment. The off-gases are transported to an incinerator. The gas and tracing temperatures as well as the nitrogen flow are computer controlled and all temperatures, pressures and flows are logged

LEGAL ASPECTS

During the torrefaction, combustible gas is formed which is intended for heat production in gas boilers or combustors. Special care must be taken to avoid leaking of gas during the process. The standard safety equipment (gas monitoring, gas pipelines, fire security) with which the lab is equipped is sufficient to conform to safety rules.

Concerning the waste stream processing, the standard personal safety equipment (gloves, glasses) is sufficient, as the streams are not dangerous or toxic.

There are no legal aspects that may create obstacles as to the operation of the waste handling and the use of products.

PRETREATMENT

The waste streams need to be placed in an oven for drying prior to their storage, to avoid decay yeast formation before their thermal treatment. Shrimp waste contains 75% water. If the material is delivered fresh, it needs to be deep frozen to avoid odor formation. Long term storage is not possible.

The untreated materials must be dry and in air tight containers, the product (biochars) need to be stored in closed vessel to avoid dust inhalation.

TESTS

Three batches of shrimp peels were roasted in the batch-reactor. First the shrimp peels were dried at 105 C overnight. Subsequently 20 liter of peels were loaded into the batch reactor. The reactor was heated to the desired temperature, being 200, 255 or 300 C. The heating up stage consisted of 1 hour. The temperature could be monitored by 3 thermocouples in the bottom, middle and top section. A continuous stream of nitrogen with 0.3% steam was applied during the whole period. The temperatures during roasting for the three tests are shown below.


Figure 1. Temperature history during the 3 tests as average values of the lower, middle and higher reactor section.  

 

The different temperature treatment gave a very distinctly different color as shown in the picture 2 below. The mass loss of the material was: 6% at 200oC, 15% at 255oC and 26% at 300oC.